I have recently joined a club and we are discussing entering a group build in an IPMS event next year.
It is generally accepted we will be doing a single engine WWII era aircraft used by multiple nations.

I have never done anything like this before and I am nervous.

I’ve been modeling for years but never showed any of them. From examples I see in magazines I have so far to go and my air brush skills are nil, although I am working on this.

In these contest how critical are colors? I.E. exact match’s of shades on the aircraft and accessories. To me OD is OD; grey is grey, but some people talk of various shades. The interior color, Zinc Chromate, Interior Green/Yellow don’t seem critical to me but will the judges feel the same? Tires - I have never found any pre prepared rubber paint that looks real; nor have I been able to make a mix that totally agrees with me. How critical are the judges?

After Market parts or parts from another model kit (whether or not the part is from the same manufacture) - are they acceptable. I have never put spark plug cables on any model; I don’t like resin but know I will have to start; I’ve never used photo etched parts and know I will have to learn how to now.

Decal location - how critical are judges going to be?

Fortunately I have a good amount of time to work on this project and I want to present a respectable product.

My greatest problem is my impatience - I’m in a hurry to get the model on the shelf and have in the past taken short cuts in the building process and accepted less than satisfactory results. I know I have to overcome this now.

You've realized already what you need to overcome, and that is rushing your build. Know that if you see an issue and think that it might dock you marks, then guaranteed a judge will see it. Take your time and keep in mind that "good enough" can always be improved upon.

I agree with Andy. It's many years since I entered any IPMS comps, and some of the rules have changed since then, obviously, due to items such as PE and resin detail parts, which hadn't even been thought of back then. But, there are still variations of the 'Out of the box' standard, as far as I know, so you don't have to throw a shed load of after market parts at a model to make it good, and a possible class winner.
The main thing is, overcoming the impatience - as Wojtek often states, modelling is not a race, and a build will take as long as it takes. Rushing to get a model finished is just rushing to lose points, when, for the sake of a few extra minutes here, or an hour there, a 'so so' model can be made into a winner and, what's more important, or at least to me anyway, the satisfaction of knowing that you like the model, and it will pass close scrutiny, instead of knowing, deep down, that it could have been 'X' percent better if more time and concentration had been invested.
Have a look at the rules Andy posted, get an idea of what's required, and then plan and execute the build in logical stages, taking your time to ensure things are done to the best of your ability, not the pre-made work of an after market 'add on' and, above all, enjoy it - it's a hobby, and a creative one at that - if you really want to have something on the shelf a couple of hours after opening the box, then buy a ready-built or die-cast model, but if you want something good on the shelf that you created, then slow down, concentrate, and enjoy the finished product in all its glory.

Research, research, research...
Back in the day alot of us didn't have access to accurate information, I refer you to my lavender/pink "Rufe", posted somewhere on this site. Even rule #7 in the painting rules doesn't excuse that!